1. Field of the Invention
The present invention relates to a magnetic carrier and a two-component developer, which are used in electrophotography, electrostatic recording, and electrostatic printing.
2. Related Background Art
Conventionally, in an image-forming apparatus using electrophotography, such as a printer or a copying machine, a two-component developer containing toner and a magnetic carrier has been suitably used from the viewpoints of image quality, durability, and high-speed response ability. The following developing method has been used as a developing method making use of such a two-component developer in order to ensure a sufficient image density and enhance fine-line reproducibility. The method includes: bringing a magnetic brush of the developer into contact with a photosensitive member; making the peripheral speed of a developing sleeve faster than that of the photosensitive member; and superimposing an alternating electric field and a direct-current electric field on each other.
A magnetic carrier used in such a contact two-component developing method is one prepared by coating the surfaces of core particles of ferrite, magnetite, or the like with an insulative resin. This is because the magnetic carrier is provided with voltage tightness to some extent or more with respect to the applied electric field. However, the magnetic carrier coated with the insulative resin is being insulated. Therefore, the carrier cannot act as a developing electrode at the time of development. As a result, the carrier may cause an image defect such as a so-called blank area having an edge effect between halftone and solid black.
For alleviating the image defect and stabilizing the electrification to toner over a long period of time, Japanese Patent Application Laid-Open No. H10-307429 proposes a magnetic carrier prepared by dispersing fine resin particles containing conductive powders in a coating resin. In addition, Japanese Patent No. 3173374 proposes a magnetic carrier prepared by dispersing both resin fine particles and a conductive material in a resin having a critical surface tension of 35 dyne/cm or less as a coating material.
Although those magnetic carriers are capable of suppressing an image defect and also preventing the contamination (soiling with spent toner) of their surfaces, ferrite particles are used as core particles of the magnetic carriers, so that a developer magnetic brush will easily cause unevenness in sweeping in contact two-component development. In this case, furthermore, toner can be degraded by the stress to the toner at the time of continuing low-consumption printing, so that a problem of making the separation of toner from a magnetic carrier worse may occur.
Japanese Patent Application Laid-Open No. H9-281807 proposes to use a magnetic material-dispersed type carrier with lowered magnetic force and increased resistance. In addition, Japanese Patent Application Laid-Open No. 2000-039740 proposes to prevent the generation of spent toner by coating the surface of a magnetic material-dispersed type carrier with a resin having an aminosilane-coupling agent and a unit such as a fluoroalkyl unit or a methylene unit.
In those methods, the specific resistance of a carrier is high, and a developing sleeve and a photosensitive member are rotated in their reverse directions to prevent the carrier having a comparatively low magnetic force from generating an image defect such as a blank area, thereby providing the carrier with a high image density and excellent dot reproducibility and alleviating the contamination of the carrier. However, when a process speed is accelerated, an increase in sliding friction force of the developer magnetic brush to the photosensitive member occurs at a developing area, so that the developer may be deteriorated. The method for development by rotating a developing sleeve and a photosensitive member in their reverse directions may cause unevenness in sweeping (hereinafter, referred to as scavenging effect) due to the sliding friction force of the magnetic brush, particularly at higher process speeds, compared with the method for development by rotating both the developing sleeve and the photosensitive member in their forward directions. In addition, the amount of development (image density) and a change in amount of development with the gradation of electric potential (gamma curve) tend to be altered by variations in distance between the developing sleeve and the photosensitive member, strength of an alternating electric field, amount of the developer carried on the developing sleeve, and so on.